1. Field of the Invention
The present invention relates to improved phototransistor Darlington devices, and to a high yield process for making such devices.
2. Description of the Prior Art
Incorporation of a phototransistor in a Darlington circuit results in a photosensitive device having relatively high output current for a certain level of incident light. This performance results since the effective gain or .beta. of the device is the product of the .beta.'s of the individual phototransistor and second transistor in the Darlington circuit. Thus, e.g., even if both transistors have the relatively low gain of .beta.=30, the overall device gain will be high, namely .beta.=900. Thus the Darlington device will have much higher output than that realizable with a single phototransistor, particularly since it is difficult to make an individual transistor with a .beta. greater than about 600.
However, the fact that the gains are multiplicative in a photo-Darlington device means that process control is more difficult. Generally such devices are batch fabricated, with a large plurality of devices being formed on a single semiconductor wafer. The phototransistor and second transistors of all the devices are formed at the same time. As a result of normal process variations certain of the transistors may have gains above or below the to tolerable limits. Because the .beta.'s of the two transistors on each device are multiplied, even a small variation in the gain of one transistor can result in a device that must be discarded as out of acceptable performance range. Low yield results.
Generally the desired device performance is specified in terms of output current for a certain incident light level. For example, it may be necessary to produce a plurality of devices each of which provides an output current of between 1.5 milliamperes minimum and 6 ma maximum when illuminated with 2 milliwatts per square centimeter of light from a tungsten source having a color temperature of 2870.degree.. Using prior art fabrication techniques, it was quite common to produce in the same wafer some devices within the desired 1.5 to 6 ma range, and other unacceptable devices which had output currents as high as 16 ma.
An object of the present invention is to provide a process for fabricating photo-Darlington devices that results in higher yields than possible using prior art techniques.
Another shortcoming of prior art photosensors is that the current output may differ if the illumination striking the device is not uniform. Thus a further object of the present invention is to provide a photo-Darlington device in which the current output is substantially the same regardless of light uniformity.